The United States of America has always enjoyed a national security and military advantage not afforded to other countries by virtue of being separated from much of the world by the Atlantic and Pacific Oceans. It has provided both safety and a temporal barrier that has given us time and space in which to plan for war.
Other than the War of 1812 and some Mexican border escapades, the US mainland has never really been threatened in a meaningful way. We have never suffered the fate of London or Paris or Berlin.
The Germans in WWII were able to bring the fight to our home waters on the Atlantic with their subs and the Japs shelled the west coast but other than that, these oceans have served us well.
The terrorists of 9-11 broke that safe barrier with their attack.
We live in a different world. That world has a resurgent traditional military threat (Russia, China) and an asymmetrical warfare threat. It has become more, not less, complicated.